1. Field of the Invention
This invention relates to the field of computer networks.
2. Background Art
Current Ethernet systems described by the ANSI/IEEE 802.3 standard typically use one of four media to connect computer equipment to a computer network: thick coax (thicknet), optical fiber, thin coax (thinnet), or twisted pair wiring (commonly referred to as 10Base-T). Twisted pair wiring is hereinafter referred to as simply 10Base-T. Of these four media, the more costly thicknet and optical fiber are mainly used for connections over long distances such as between buildings. Less expensive thinnet and 10Base-T are used to connect computer equipment including personal computers (PC), workstations, etc. Thinnet and 10Base-T each have their own advantages and disadvantages.
A local area network using thinnet is constructed by connecting segments of coaxial cable together forming a bus topology using either custom-cut lengths of cable having crimped-on baby N connectors (BNC) or fixed length pre-made lengths of cable. At each end of the assembled cable is a terminator. A connection to the cable is made via a T connector that is inserted between two cable segments. The T connector attaches to computer equipment, referred to as Data Terminal Equipment (DTE) in IEEE Standard 802.3, using a Medium Attachment Unit (MAU). The MAU should be a simple, inexpensive, and flexible means of attaching computer equipment to the media which, in this case, is coaxial cable. The MAU converts signals on coaxial cable into a standard electrical interface referred to as an Attachment Unit Interface (AUI) which is common to nearly all Ethernet equipment.
IEEE Standard 802.3 for 10Base-2 Ethernet defines the functional, electrical, and mechanical specifications for a thinner MAU. A thinnet MAU handles message flow between computer equipment and a thinnet bus. The MAU must be able to transmit streams of serial data from computer equipment to the thinnet bus, receive streams of serial data from the thinnet bus and transmit the data to computer equipment, detect collisions on the network due to another MAU transmitting concurrently with the local MAU, and automatically interrupt the transmit function to inhibit a long data stream. An optional monitor function disables the transmit function and prevents improper loading of the bus due to the disabled transmitter while permitting computer equipment to continue receiving data and detect collisions.
FIG. 1 is a diagram illustrating a method of adding computer equipment to a network having a bus topology implemented with coaxial cable (i.e. thinnet). The diagram shows a network segment comprising T connectors 106 and 108, terminator 110 and MAUs 102 and 104 having AUIs 130 and 132, respectively. AUIs 130 and 132 are interfaces to computer equipment. A thinnet cable 120 is coupled between a previous thinnet node (not shown in FIG. 1) and T connector 106. T connector 106 is coupled to MAU 102 and to thinnet cable 122. Thinnet cable 122 is coupled to T connector 108. T connector 108 is coupled to MAU 104 and to thinnet cable 124. Thinnet cable 124 is coupled to terminator 110.
A typical network begins with one cable segment 120 connecting two PCs or computer equipment. MAU 102 is coupled to the second computer equipment. When a third PC or item of computer equipment having AUI 132 coupled to MAU 104 is added to the network, a terminator 110 is removed from an end of cable segment 122, a new T connector 108 is attached between the aforesaid cable 122 and a new segment of cable 124, and terminator 110 is placed on the end of new cable segment 124 as illustrated in FIG. 1. Additional PCs or computer equipment may be added in a similar fashion. This method of wiring is commonly referred to as daisy-chaining.
Thinnet has the advantage of being easy to install and allowing easy growth of a network. No external equipment other than cable and MAUs is required to construct a network. In addition, cables and MAUs can be acquired incrementally: for each additional node added to the network, one thinnet MAU, one T connector, and one additional segment of coaxial cable are needed. Attaching additional nodes to an existing network is as simple as purchasing a MAU, a T connector, and a segment of coaxial cable for each node, and then stringing these items on either end of the existing cable in the manner described above.
The disadvantage of using thinnet becomes apparent as the network grows. Coaxial cable cannot be easily routed to all parts of a building, especially when the cable must remain connected together as a continuous segment. In addition, if the cable should break or be disconnected at any point, the two resulting halves become useless because they are not terminated at the break. Due to thinnet's bus topology and the lack of diagnostic capability built into the MAUs, finding the location where the break has occurred is not easily accomplished without specialized equipment (such as a time domain reflectometer), that the user is unlikely to have.
A competing wiring system, 10Base-T, builds a network as illustrated in FIG. 2A comprising an active hub 200 and nodes 202, 204 and 206. A port 200A of active hub 200 is coupled to node 202 by two 10Base-T twisted pairs 208 comprising transmit pair 208A and receive pair 208B. Port 200B of active hub 200 is coupled to node 204 by two 10Base-T twisted pairs 210 comprising transmit pair 210A and receive pair 210B. Port 200C of active hub 200 is coupled to node 206 by two 10Base-T twisted pairs 212 comprising transmit pair 212A and receive pair 212B.
The network illustrated in FIG. 2A comprises an external active hub 200 that is centrally located to connect to each node 202, 204 and 206 over standard telephone twisted pair wiring 208, 210 and 212. Each node 202, 204 and 206 requires its own separate connection to hub 200 using two pairs 208, 210 and 212 of wire (one pair 208A, 210A and 212A for transmitting signals and the other pair 208B, 210B and 212B for receiving signals), respectively. At nodes 202, 204 and 206, computer equipment connects to a port 200A-200C of an active hub 200, respectively, using a 10Base-T MAU thereby forming a point-to-point link between an active hub 200 and DTEs. The hub 200 consists of a reclocking repeater that has a separate port 200A-200C for each node connected to it. If another node is added to the network, the network requires two pairs of wire connecting the node to its own port on the hub. This type of wiring is usually referred to as either a star-topology or structured wiring, and is used by most telephone systems.
IEEE Standard 802.3 for 10Base-T Twisted Pair Ethernet defines the functional, electrical, and mechanical specifications for the 10Base-T MAU and the medium. A 10Base-T MAU handles message flow between computer equipment or hub (repeater) and a twisted pair link. The MAU must be able to transmit streams of serial data from computer equipment to the twisted pair link, receive streams of serial data from the twisted pair link and transmit data to computer equipment, detect collisions on the network due to another MAU transmitting concurrently with the local MAU, verify that the MAU and AUI are connected, automatically interrupt the transmit function by inhibiting a long data stream, and to test and disable a receive link if the link is faulty.
The chief disadvantage of 10Base-T is that it requires an external hub, each port of which can only connect to one node. However, one of the main advantages of using 10Base-T medium is avoiding the cost and inconvenience of routing cable to each node since telephone-grade twisted pair wiring already exists in most buildings and is therefore easier to use than coaxial cable or optical fiber. Since 10Base-T requires 4 wires (two twisted wire pairs) for each node, the supply of existing wiring in a building may be used up quickly. If two nodes are located in a room that only has one 10Base-T connection (two pairs of wires), two additional pairs of wires must be routed to the location to connect the second node to the hub, thus defeating one of the advantages of using existing twisted pair wiring.
In addition, each node added uses up an additional port on a hub, thereby requiring the installer to purchase an appropriate number of hubs to match the number of nodes. If the number of nodes exceeds the number of ports on a hub, an additional hub must be purchased to accommodate the nodes and the expense of the remaining unused ports is lost. For installations that have standardized on 10Base-T, the increasing number of nodes that must be added to the network becomes a major problem. Current alternative methods of adding nodes to a 10Base-T also have their difficulties.
FIG. 2B is a block diagram illustrating a 10Base-T network comprising active hub 230, mini-hub 240 and nodes 232, 234, 236 and 238 that uses 10Base-T mini-hub 240 to add nodes 236 and 238 to network. Ports 230A-230C of active hub 230 are coupled to nodes 232 and 234 and mini-hub 240 by 10Base-T twisted wire pairs 242, 244 and 246, respectively. Each 10Base-T twisted wire pairs 242, 244 and 246 comprises transmit pair 242A, 244A and 246A and receive pair 242B, 244B and 246C, respectively. Ports 240A and 240B of mini-hub 240 are coupled to nodes 236 and 238 by two twisted wire pairs 248 and 250 comprising transmit pair 248A and 250A and receive pair 248B and 250B, respectively.
If an additional two pairs of wire and an unused hub port are not available in a network as shown in FIG. 2A, a 10Base-T mini hub 240 can be connected to the main network as illustrated in FIG. 2B, and nodes 236 and 238 can then be connected to two ports 240A and 240B of the added mini hub 240. In this manner, a node comprising mini-hub 240 coupled to port 230C of active hub 230 connects nodes 236 and 238 to the network. However, this dramatically increases the incremental cost of adding a single node since an entire hub must be added. In addition, the Ethernet specification limits the number of hubs that can be between nodes, so adding an additional hub may not even be possible without violating the Ethernet standard.
FIG. 2C is a block diagram comprising active hub 260, nodes 262-270, and a 10Base-T to thinnet converter 272 that illustrates another common method of adding a node to a network. Ports 260A-260C of active hub 260 are coupled to nodes 262 and 264 and to 10Base-T to thinnet converter 272 by 10Base-T twisted wire pairs 274, 276 and 278, respectively. Each 10Base-T twisted wire pairs 274, 276 and 278 comprises transmit pair 274A, 276A and 278A and receive pair 274B, 276B and 278C, respectively. A thinnet bus 280 couples nodes 266, 268 and 270 to 10Base-T to thinnet converter 272 in a daisy-chain manner.
10Base-T to thinner converter 272 allows nodes 266, 268 and 270 to be daisy-chained together using thinnet cable, and then to connect the thinnet segment 280 to a main 10Base-T network comprising active hub 260 and nodes 262 and 264 through converter 272. Again, this method has the disadvantage of requiring an external device (10Base-T to thinnet converter 272) to expand a single 10Base-T port 260C of active hub 260 in order to connect to multiple nodes 266-270. It is also not an incremental addition, since the node that is initially a 10Base-T port must first be converted to a thinnet port.